Reflective mask blank for euv lithography

ABSTRACT

To provide a reflective mask blank for EUV lithography having an absorber layer which has a low reflectance in a wavelength region of EUV light or light for inspection of a pattern and which is easy to control to have a desired layer composition and thickness. 
     A reflective mask blank for EUV lithography, which comprises a substrate, and a reflective layer to reflect EUV light and an absorber layer to absorb EUV light, formed in this order on the substrate, wherein the absorber layer contains tantalum (Ta) and hafnium (Hf), and in the absorber layer, the content of Hf is from 20 to 60 at. % and the content of Ta is from 40 to 80 at. %.

TECHNICAL FIELD

The present invention relates to a reflective mask blank (in thisspecification, hereinafter referred to as “EUV mask blank”) for EUV(Extreme Ultra Violet) lithography to be used for e.g. production ofsemiconductors.

BACKGROUND ART

In the semiconductor industry, a photolithography method using visiblelight or ultraviolet light has been employed as a technique for writing,on a Si substrate or the like, a fine pattern, which is required forforming an integrated circuit comprising such a fine pattern. However,the conventional photolithography method has been close to theresolution limit, while microsizing of semiconductor devices has beenaccelerated. In the case of the photolithography method, it is said thatthe resolution limit of a pattern is about ½ of an exposure wavelength,and that even if an immersion method is employed, the resolution limitis about ¼ of an exposure wavelength. Even if an immersion method usingan ArF laser (193 nm) is employed, it is estimated that the resolutionlimit is about 45 nm. From this point of view, EUV lithography, which isan exposure technique using EUV light having a shorter wavelength thanArF lasers, is considered to be promising as an exposure technique for45 nm or below. In this specification, “EUV light” means a ray having awavelength in a soft X-ray region or a vacuum ultraviolet ray region,specifically a ray having a wavelength of from about 10 to 20 nm, inparticular, of about 13.5 nm±0.3 nm.

EUV light is apt to be absorbed by any substances and the refractiveindices of substances are close to 1 at this wavelength, whereby it isimpossible to use a dioptric system like a conventional photolithographyemploying visible light or ultraviolet light. For this reason, for EUVlight lithography, a catoptric system, i.e. a combination of areflective photomask and a mirror, is employed.

A mask blank is a stacked member for fabrication of a photomask, whichhas not been patterned yet. In the case of an EUV mask blank, it has astructure wherein a substrate made of glass or the like has a reflectivelayer to reflect EUV light and an absorber layer to absorb EUV light,formed thereon in this order. As the reflective layer, a multilayerreflective film is usually used wherein high refractive index layers andlow refractive index layers are alternately stacked to increase a lightreflectance when irradiating the layer surface with EUV light. For theabsorber layer, it is common to employ a material having a highabsorption coefficient for EUV light, specifically e.g. a materialcontaining Cr or Ta as the main component.

Patent Document 1 discloses that a nitride of a tantalum/boron alloy(TaBN), an oxide of a tantalum/boron alloy (TaBO) and an oxynitride of atantalum/boron alloy (TaBNO) have high absorption coefficients for EUVlight, and further have a low reflectance against deep-ultraviolet lightwithin a wavelength region (from 190 nm to 260 nm) of light forinspection of a pattern, and thus, they are preferred as materials foran absorber layer.

Further, Patent Documents 1 and 2 disclose that in order to make theabsorber layer surface to be a surface excellent in flatness andsmoothness, the crystalline state of the absorber layer is preferablyamorphous, and in order to make the crystalline state of a TaBN film, aTaBO film and a TaBNO film to be amorphous, the content of B in suchfilms is preferably from 5 to 25 at. %.

Patent Document 1: JP-A-2004-6798

Patent Document 2: JP-A-2004-6799

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in a case where the absorber layer is made of a TaBO film or aTaBNO film, if the content of O in the film increases, the insulatingproperty of the absorber layer increases, and at the time of electronbeam writing in such an absorber layer, charge-up takes place, suchbeing undesirable.

On the other hand, in a case where the absorber layer is made of a TaBNfilm, there will be no substantial possibility that charge-up will occurat the time of electron beam writing.

In a case where the absorber layer is to be made of a TaBN film, filmdeposition is carried out in many cases by using a magnetron sputteringmethod as a method whereby defects are less likely to result. In such acase, for example, a Ta target and a B target may be used, and suchtargets may be discharged at the same time in a nitrogen atmosphere toform a TaBN film. Otherwise, a TaB compound target may be used, and sucha compound target may be discharged in a nitrogen atmosphere to form aTaBN film.

However, for example, in the case of the method of using a Ta target anda B target, the B target has a high resistance and is a light elementand thus its film deposition rate is at most 1/10 as compared with theTa target in many cases. Therefore, in order to attain the content (atleast 5 at. %) of B required to make the crystalline state of the filmto be amorphous, as disclosed in Patent Document 1, the film depositionrate of the Ta target is required to be low, such being undesirable asthe production efficiency decreases substantially.

On the other hand, in the method of using the TaB compound target, if acompound target comprising 20 at. % of B and 80 at. % of Ta, is, forexample, used, the maximum content of B to be actually incorporated inthe film will be at a level of 6 at. %, whereby it will be difficult tocontrol the content of B in the film to be at least 5 at. %. Further, ifN is added, the content of B in the film tends to be at most 4 at. %,whereby the crystalline state of the film cannot be made to beamorphous.

In order to solve this problem, it is conceivable to further increasethe B content in the TaB compound target (for example, B is made to be50 at. %, and Ta is made to be 50 at. %) to increase the content of B inthe film. However, as the content of B in the TaB target increases, thedensity of the target becomes low, whereby the processability becomespoor. Further, the electrical resistance of the TaB target increases,whereby the discharge tends to be unstable, and the film deposition ratetends to be slow. If the discharge becomes unstable, the composition orthickness of the film tends to be non-uniform, and in some cases, filmdeposition may become impossible.

It is an object of the present invention to solve the above-describedproblems of the prior art and to provide an EUV mask blank which isexcellent in the characteristics as an EUV mask blank and which has anabsorber layer which has a particularly low reflectance within awavelength region of EUV light and light for inspection of a pattern andbeing easily controlled to have a desired film composition and filmthickness.

Means to Solve the Problems

The present inventors have conducted an extensive study to solve theabove problems and as a result, have found that when the absorber layeris made of a film (a TaHf film) comprising Ta and Hf, the crystallinestate of the film becomes amorphous, and it is possible to obtain anabsorber layer excellent in etching characteristics and opticalcharacteristics. They have further found that when such a TaHf film isto be used as an absorber layer, it can be produced under a stabilizedcondition without bringing about deterioration of the film depositionrate.

The present invention has been made on the basis of such discoveries andprovides a reflective mask blank for EUV lithography, which comprises asubstrate, and a reflective layer to reflect EUV light and an absorberlayer to absorb EUV light, formed in this order on the substrate,wherein the absorber layer comprises tantalum (Ta) and hafnium (Hf), andin the absorber layer, the content of Hf is from 20 to 60 at. % and thecontent of Ta is from 40 to 80 at. %.

Further, the present invention provides a reflective mask blank for EUVlithography, which comprises a substrate, and a reflective layer toreflect EUV light and an absorber layer to absorb EUV light, formed inthis order on the substrate, wherein the absorber layer comprisestantalum (Ta), hafnium (Hf) and nitrogen (N), and in the absorber layer,the total content of Ta and Hf is from 40 to 70 at. %, the compositionalratio of Ta to Hf is Ta:Hf=8:2 to 4:6, and the content of N is from 30to 60 at. %.

In the above absorber layer, the compositional ratio of Ta to Hf ispreferably from 7:3 to 4:6.

In the EUV mask blank of the present invention, the above absorber layeris preferably such that the total content of B, Si and Ge is from 0 toat most 5 at. %.

In the EUV mask blank of the present invention, the above absorber layermay contain from 0.1 to 1.0 at. % of Zr.

In the EUV mask blank of the present invention, the crystalline state ofthe above absorber layer is preferably amorphous.

Further, in the EUV mask blank of the present invention, the aboveabsorber layer preferably has a surface roughness (rms) of at most 0.5nm.

Further, in the EUV mask blank of the present invention, the aboveabsorber layer preferably has a thickness of from 50 to 200 nm.

The EUV mask blank of the present invention is preferably such that alow reflective layer against an inspection light to be used forinspection of a mask pattern is formed on the absorber layer, the lowreflective layer comprises tantalum (Ta), hafnium (Hf) and oxygen (O),and in the low reflective layer, the total content of Ta and Hf is from30 to 80 at. %, the compositional ratio of Ta to Hf is from 8:2 to 4:6,and the content of O is from 20 to 70 at. %.

In the above low reflective layer, the compositional ratio of Ta to Hfis Ta:Hf=7:3 to 4:6.

Further, the EUV mask blank of the present invention is preferably suchthat a low reflective layer against an inspection light to be used forinspection of a mask pattern is formed on the absorber layer, the lowreflective layer comprises tantalum (Ta), hafnium (Hf), oxygen (O) andnitrogen (N), and in the low reflective layer, the total content of Taand Hf is from 30 to 80 at. %, the compositional ratio of Ta to Hf isTa:Hf=8:2 to 4:6, the total content of N and O is from 20 to 70 at. %,and the compositional ratio of N to O is N:O=9:1 to 1:9.

In the above low reflective layer, the compositional ratio of Ta to Hfis preferably from 7:3 to 4:6.

Further, the present invention provides a reflective mask blank for EUVlithography, which comprises a substrate, and a reflective layer toreflect EUV light, an absorber layer to absorb EUV light and a lowreflective layer against an inspection light to be used for inspectionof a mask pattern, formed in this order on the substrate, wherein thelow reflective layer comprises tantalum (Ta), hafnium (Hf) and oxygen(O), and in the low reflective layer, the total content of Ta and Hf isfrom 30 to 80 at. %, the compositional ratio of Ta to Hf is from 8:2 to4:6, and the content of O is from 20 to 70 at. %.

In the above low reflective layer, the compositional ratio of Ta to Hfis Ta:Hf=7:3 to 4:6.

Further, the present invention provides a reflective mask blank for EUVlithography, which comprises a substrate, and a reflective layer toreflect EUV light, an absorber layer to absorb EUV light and a lowreflective layer against an inspection light to be used for inspectionof a mask pattern, formed in this order on the substrate, wherein thelow reflective layer comprises tantalum (Ta), hafnium (Hf), oxygen (O)and nitrogen (N), and in the low reflective layer, the total content ofTa and Hf is from 30 to 80 at. %, the compositional ratio of Ta to Hf isTa:Hf=8:2 to 4:6, the total content of N and O is from 20 to 70 at. %,and the compositional ratio of N to O is N:O=9:1 to 1:9.

In the above low reflective layer, the compositional ratio of Ta to Hfis preferably Ta:Hf=7:3 to 4:6.

In a case where the low reflective layer is formed on the absorberlayer, the low reflective layer preferably contains from 0.1 to 1.0 at.% of Zr.

Further, in a case where the low reflective layer is formed on theabsorber layer, the low reflective layer preferably has a surfaceroughness (rms) of at most 0.5 nm.

Further, in a case where the low reflective layer is formed on theabsorber layer, the low reflective layer preferably has a thickness offrom 5 to 30 nm.

Further, the EUV mask blank of the present invention is preferably suchthat a protective layer to protect the reflective layer during formationof a pattern in the absorber layer, is formed between the reflectivelayer and the absorber layer, and the contrast between light reflectedon the surface of the protective layer and light reflected on thesurface of the low reflective layer at a wavelength of light to be usedfor inspection of the pattern formed in the absorber layer, is at least30%.

In a case where the protective layer is formed between the reflectivelayer and the absorber layer, the protective layer is preferably formedof any one selected from the group consisting of Ru, a Ru compound, SiO₂and CrN.

In a case where the low reflective layer is formed on the absorberlayer, the reflectance on the surface of the low reflective layer at awavelength of light to be used for inspection of the pattern formed inthe absorber layer, is at most 15%.

The EUV mask blank of the present invention is preferably such that theabove absorber layer is formed by carrying out a sputtering method usinga target made of a TaHf compound.

Further, in the EUV mask blank of the present invention wherein theabsorber layer comprises tantalum (Ta), hafnium (Hf) and nitrogen (N),the above absorber layer is preferably formed by carrying out asputtering method using a target made of a TaHf compound in anatmosphere containing nitrogen.

Here, the target made of a TaHf compound preferably has a compositioncomprising Ta=30 to 70 at. % and Hf=70 to 30 at. %.

Further, the target made of a TaHf compound may contain from 0.1 to 5.0at. % of Zr.

In a case where the low reflective layer comprising tantalum (Ta),hafnium (Hf) and oxygen (O) is to be formed on the absorber layer, theabove low reflective layer is preferably formed by carrying out asputtering method using a target made of a TaHf compound in anatmosphere containing oxygen.

Further, in a case where the low reflective layer comprising tantalum(Ta), hafnium (Hf), oxygen (O) and nitrogen (N) is to be formed on theabsorber layer, the above low reflective layer is preferably formed bycarrying out a sputtering method using a target made of a TaHf compoundin an atmosphere comprising nitrogen and oxygen.

Here, the target made of a TaHf compound preferably has a compositioncomprising Ta=30 to 70 at. % and Hf=70 to 30 at. %.

Further, the target made of a TaHf compound may contain from 0.1 to 5.0at. % of Zr.

EFFECTS OF THE INVENTION

In the EUV mask blank of the present invention, the absorber layercomprises Hf having a low electrical resistivity, and accordingly, it isfree from deterioration of the film deposition rate during the filmdeposition for the absorber layer and free from instability of thedischarge during the film deposition, which may occur in the case wherethe absorber layer has high electrical resistance and the absorber layercontains insulating B (boron). Accordingly, there will be no suchproblem that the film composition or the film thickness fluctuates atthe time of forming the absorber layer or further that the filmdeposition becomes impossible.

In the EUV mask blank of the present invention, the crystalline state ofthe absorber layer is amorphous, and the absorber surface is excellentin smoothness. As a result, there will be no such a trouble that theedge roughness of a pattern formed in the absorber layer becomes largeor that the dimensional precision of the pattern deteriorates.

Further, the absorber layer has characteristics excellent for an EUVmask blank, such that the reflectance against EUV light and thereflectance against light within a wavelength region of light to be usedfor inspection of a pattern, are low.

Further, the absorber layer comprising Ta and Hf has an etching ratehigher than a TaBN film, whereby an effect to reduce the damage to theresist during etching, is expected. Further, by such reduction of thedamage to the resist, it is expected that the film thickness of theresist can be reduced.

In the EUV mask blank of the present invention, by forming a lowreflective layer on the absorber layer, it is possible to furthersuppress the reflectance against light in a wavelength region of lightto be used for inspection of a pattern, and the contrast will be good atthe time of inspection of a pattern which is carried out after forming apattern on the mask blank. Further, since the crystalline structure ofthe low reflective layer is amorphous, the absorber surface is excellentin smoothness. As a result, the edge roughness of the pattern to beformed in the absorber layer will not increase, and the dimensionalprecision of the pattern will not deteriorate.

In the EUV mask blank of the present invention, when the absorber layerand the low reflective layer are to be formed by a sputtering method, byusing a TaHf compound target having a specific composition, it ispossible to avoid instability of the discharge or fluctuation in thefilm composition or film thickness.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating one embodimentof the EUV mask blank of the present invention.

FIG. 2 illustrates a state wherein a pattern was formed in an absorberlayer 14 (and a low reflective layer 15) of the EUV mask blank 1 shownin FIG. 1.

MEANINGS OF SYMBOLS

-   -   1: EUV mask blank    -   11: Substrate    -   12: Reflective layer (multilayer reflective film)    -   13: Protective layer    -   14: Absorber layer    -   15: Low reflective layer

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the EUV mask blank of the present invention will be described withreference to the drawings.

FIG. 1 is a schematic cross-sectional view illustrating one embodimentof the EUV mask blank of the present invention. In the mask blank 1shown in FIG. 1, a reflective layer 12 to reflect EUV light and anabsorber layer 14 to absorb EUV light are formed in this order on asubstrate 11. Between the reflective layer 12 and the absorber layer 14,a protective layer 13 to protect the reflective layer 12 duringformation of a pattern in the absorber layer 14, is formed. On theabsorber layer 14, a low reflective layer 15 against an inspection lightto be used for inspection of a mask pattern is formed. However, in theEUV mask blank 1 of the present invention, in the construction shown inFIG. 1, only the substrate 11, the reflective layer 12 and the absorberlayer 14 are essential, and the protective layer 13 and the lowreflective layer 15 are optional constituting elements. Further, the EUVmask blank of the present invention may have other layers formed inaddition to the above layers.

Now, the individual constituting elements of the mask blank 1 will bedescribed.

The substrate 11 is required to satisfy the characteristics as asubstrate for an EUV mask blank. Therefore, the substrate 11 ispreferably one which has a low thermal expansion coefficient(specifically the thermal expansion coefficient at 20° C. is preferably0±0.05×10⁻⁷/° C., particularly preferably 0±0.03×10⁻⁷/° C.) and which isexcellent in smoothness, flatness and resistance against a cleaningliquid to be used for e.g. cleaning of a mask blank or a photomask afterformation of a pattern. Specifically, as such a substrate 11, glasshaving a low thermal expansion coefficient, such as a SiO₂—TiO₂ glass,may, for example, be used, but the substrate is not limited thereto, anda substrate of e.g. crystallized glass having β-quartz solid solutionprecipitated, quartz glass, silicon or metal, may also be used.

The substrate 11 preferably has a smooth surface with a surfaceroughness (rms) of at most 0.15 nm and a flatness of at most 100 nm,whereby a high reflectance and transfer precision can be obtained with aphotomask after formation of a pattern.

The size, thickness, etc. of the substrate 11 are suitably determineddepending upon the design values of the mask. In Examples shownhereinafter, a SiO₂—TiO₂ glass having a size of 6 inch (152 mm) squareand a thickness of 0.25 inch (6.3 mm) was used.

It is preferred that no defects are present on the surface of thesubstrate 11 on the side on which the reflective layer 12 is to beformed. Even if they are present, in order that no phase defects willform by concave defects and/or convex defects, the depth of the concavedefects and the height of the convex defects are preferably at most 2nm, and the half value widths of such concave defects and convex defectsare preferably at most 60 nm.

The reflective layer 12 is not particularly restricted so long as it isone having desired characteristics as a reflective layer for an EUV maskblank. Here, the characteristic particularly required for the reflectivelayer 12 is a high EUV light reflectance. Specifically, when the surfaceof the reflective layer 12 is irradiated at an incident angle of 6° withlight in a wavelength region of EUV light, the maximum value of thelight reflectance in the vicinity of a wavelength of 13.5 nm ispreferably at least 60%, more preferably at least 65%. Even in a casewhere a protective layer 13 or a low reflection layer 15 is formed onthe reflection layer 12, the maximum value of the light reflectance inthe vicinity of a wavelength of 13.5 nm is preferably at least 60%, morepreferably at least 65%.

As the reflective layer 12, a reflective multilayer film having highrefractive index layers and low refractive index layers alternatelystacked in a plurality of times, is usually used as the reflective layer12, whereby a high EUV light reflectance can be attained. In thereflective multilayer film constituting the reflective layer 12, Mo iswidely used for the high refractive index layers, and Si is widely usedfor the low refractive index layers. Namely, a reflective Mo/Simultilayer film is most common. However, the reflective multilayer filmis not limited thereto, and a reflective Ru/Si multilayer film, areflective Mo/Be multilayer film, a reflective Mo compound/Si compoundmultilayer film, a reflective Si/Mo/Ru multilayer film, a reflectiveSi/Mo/Ru/Mo multilayer film or a reflective Si/Ru/Mo/Ru multilayer filmmay also be used.

The thicknesses and the number of repeating layer units of therespective layers constituting the reflective multilayer filmconstituting the reflective layer 12 may suitably be selected dependingupon the film materials to be used and the EUV light reflectancerequired for the reflective layer. When a reflective Mo/Si multilayerfilm is taken as an example, in order to form a reflective layer 12having the maximum value of the EUV light reflectance being at least60%, the reflective multilayer film may be formed by stacking Mo layershaving a thickness of 2.3±0.1 nm and Si layers having a thickness of4.5±0.1 nm so that the number of repeating units will be from 30 to 60.

Suitable reflective layers may be prepared by conventional methods knownto those of ordinary skill in the art without undue experimentation.

Here, the respective layers constituting the reflective multilayer filmas the reflective layer 12 may be formed to have the desired thicknessby means of a well-known film deposition method such as magnetronsputtering or ion beam sputtering. For example, in a case where areflective Si/Mo multilayer film is formed by means of ion beamsputtering, it is preferred that a Si film is deposited to have athickness of 4.5 nm at an ion-accelerated voltage of from 300 to 1,500 Vat a film deposition rate of from 0.03 to 0.30 nm/sec by using a Sitarget as the target and using an Ar gas (gas pressure: 1.3×10⁻² Pa to2.7×10⁻² Pa) as the sputtering gas, and then a Mo film is deposited tohave a thickness of 2.3 nm at an ion-accelerated voltage of from 300 to1,500 V at a film deposition rate of from 0.03 to 0.30 nm/sec by using aMo target as the target and using an Ar gas (gas pressure: 1.3×10⁻² Pato 2.7×10⁻² Pa) as the sputtering gas. This operation is regarded as onecycle, and by stacking Si films and Mo films for 40 to 50 cycles, areflective Si/Mo multilayer film will be formed.

In order to prevent oxidation of the surface of the reflective layer 12,the uppermost layer of the reflective multilayer film constituting thereflective layer 12 is preferably a layer made of a hardly oxidizablematerial. The layer made of a hardly oxidizable material will functionas a cap layer of the reflective layer 12. As a specific example of thelayer made of a hardly oxidizable material functioning as cap layer, aSi layer may, for example, be mentioned. In a case where the reflectivemultilayer film constituting the reflective layer 12 is a Si/Mo film,the uppermost layer may be made to be a Si layer, so that the uppermostlayer will function as a cap layer. In such a case, the thickness of thecap layer is preferably 11±2 nm.

The protective layer 13 is provided for the purpose of protecting thereflective layer 12, so that the reflective layer 12 will not receive adamage by an etching process at the time of forming a pattern in theabsorber layer 14 by an etching process, usually by a dry etchingprocess. Accordingly, as the material for the protective layer 13, amaterial is selected which is hardly susceptible to an influence by theetching process of the absorber layer 14, i.e. a material having anetching rate slower than the absorber layer 14 and yet is hardlysusceptible to a damage by such an etching process. A material whichsatisfies such conditions, may, for example, be Cr, Al, Ta or theirnitrides, Ru or a Ru compound (such as RuB or RuSi) as well as SiO₂,Si₃N₄, Al₂O₃ or a mixture thereof. Among them, Ru or a Ru compound (suchas RuB or RuSi), CrN or SiO₂ is preferred, and Ru or a Ru compound (suchas RuB or RuSi) is particularly preferred from the viewpoint of theadhesion or absorption property.

The thickness of the protective layer 13 is preferably from 1 to 60 nm.

The protective layer 13 is formed by using a well-known film depositionmethod such as magnetron sputtering or ion beam sputtering. In a casewhere a Ru film is formed by magnetron sputtering, it is preferred tocarry out film deposition at an applied electric power of from 30 W to500 W at a film deposition rate of from 5 to 50 nm/min so that thethickness will be from 2 to 5 nm by using a Ru target as the target andusing an Ar gas (gas pressure: 1.0×10⁻¹ Pa to 10×10⁻¹ Pa) as thesputtering gas.

The characteristic particularly required for the absorber layer 14 isthat the EUV light reflectance is very low. Specifically, when thesurface of the absorber layer 14 is irradiated with light in awavelength region of EUV light, the maximum light reflectance in thevicinity of a wavelength of 13.5 nm is preferably at most 0.5%, morepreferably at most 0.1%.

In order to attain the above characteristic, the absorber layer ispreferably made of a material having a high absorption coefficient ofEUV light.

The absorber layer 14 of the EUV mask blank 1 of the present inventioncomprises tantalum (Ta) and hafnium (Hf) in a specific ratio which willbe described hereinafter, whereby the above characteristic is attained.

The content of Hf in the absorber layer 14 is from 20 to 60 at. %. Ifthe content of Hf in the absorber layer 14 is less than 20 at. %, thecrystalline state of the absorber layer 14 tends to hardly becomeamorphous. If the content of Hf in the absorber layer 14 exceeds 60 at%, the etching characteristics of the absorber layer tends todeteriorate, and it becomes difficult to satisfy the required etchingselective ratio.

In the EUV mask blank of the present invention, the Hf content in theabsorber layer 14 is within the above range, whereby the crystallinestate of the absorber layer tends to be readily amorphous and theabsorber surface will be excellent in smoothness. Further, the absorberlayer 14 has excellent characteristics for an EUV mask blank, such thatthe EUV light reflectance and the light reflectance in the wavelengthregion of light for inspection of a pattern, are low.

The content of Hf in the absorber layer 14 is more preferably from 30 to50 at. %, further preferably from 30 to 45 at %.

Patent Document 1 mentions Hf as an example of a metal element containedin an upper layer and a lower layer in an double-layered absorbercomprising, as the lower layer, an absorber layer constituted by anabsorber for an exposure light in a short wavelength region includingextreme ultraviolet wavelength and, as the upper layer, a low reflectivelayer constituted by an absorber for an inspection light to be used forinspection of a mask pattern. However, the structure containing Ta andHf is not disclosed at all. Further, Patent Document 1 discloses thatwhen the absorber layer is made to be a TaBN film, a TaBO film or aTaBNO film, the crystalline state becomes amorphous, but there is nodisclosure that when Hf is incorporated, the crystalline state becomesamorphous.

Further, it is widely known that a metal crystal can be made to beamorphous by mixing an element such as B or Si, and in Patent Document1, such a technique is used to make the absorber layer to be amorphousthereby to make the surface smooth. However, it is not known that a filmcontaining two metal elements of Ta and Hf at the same time, can be madeto be amorphous, and also in Patent Document 1, Ta and Hf are mentionedmerely as examples of many metal elements which may be contained in theabsorber layer.

According to the present invention, the crystalline state of theabsorber layer can be made to be amorphous without using an elementknown to contribute to conversion of a conventional metal crystal to beamorphous, such as B or Si. Further, in addition to B and Si, Ge may bementioned as an element known to contribute to conversion ofconventional metal crystal to be amorphous. Such an element is one whichcontributes to conversion of a metal crystal to be amorphous, but whenincorporated to the absorber layer, an unavoidable problem will alsoresult. For example, when B is incorporated, the electrical resistanceof the target to be used for the film deposition tends to be large,whereby the discharge tends to be unstable, and at the same time, thefilm deposition rate tends to be low. If the discharge becomes unstable,there will be a problem such that the composition or thickness of thefilm tends to be non-uniform, and in some cases, there will be a problemsuch that the film deposition will be impossible. Further, if Si isincorporated, the EUV absorption coefficient of Si is small, and therewill be a problem such that the EUV light absorption characteristic ofthe absorber layer tends to be low.

Therefore, it is preferred that the absorber layer 14 does notsubstantially contain such B, Si and Ge elements, and the total contentof such B, Si and Ge is preferably 0 to at most 5 at. %. The totalcontent of such elements is more preferably 0 to at most 4 at. %,further preferably 0 to at most 3 at. %.

In the absorber layer 14, the rest excluding Hf is preferably Ta.Accordingly, the content of Ta in the absorber layer 14 is preferablyfrom 40 to 80 at. %. The content of Ta in the absorber layer 14 is morepreferably from 50 to 70 at %, further preferably from 55 to 70 at %.

Further, the absorber layer 14 may contain an element other than Ta andHf, as the case requires. In such a case, the element to be incorporatedto the absorber layer 14 is required to satisfy suitability for a maskblank, such as the absorption characteristic for EUV light.

Nitrogen (N) may be mentioned as an example of the element which may beincorporated to the absorber layer 14. In a case where the absorberlayer 14 is of a fine crystalline structure, by the incorporation of N,it is possible to further reduce the crystal particle sizes, and theincorporation is effective to improve the smoothness of the surface ofthe absorber layer 14. Further, by the incorporation of N to theabsorber layer 14, an effect to reduce a stress formed in the absorberlayer 14, is also expected.

In a case where the absorber layer 14 contains N, the total content ofTa and Hf in the absorber layer 14 is preferably from 40 to 70 at. %,and the compositional ratio of Ta to Hf is preferably Ta:Hf=8:2 to 4:6.Here, in the present invention, the compositional ratio of Ta to Hfmeans a compositional ratio of Ta to Hf by atomic ratio. If the totalcontent of Ta and Hf is less than 40 at. %, the EUV light reflectancecannot be sufficiently lowered. If the total content of Ta and Hfexceeds 70 at. %, the effects to improve smoothness and to reduce thestress, tend to be small. Further, if Hf is lower than the abovecompositional ratio, the crystalline state of the absorber layer 14tends to be hardly amorphous. If Hf in the absorber layer 14 is higherthan the above compositional ratio, the etching characteristic of theabsorber layer tends to deteriorate, and the required etching selectiveratio tends to be hardly satisfied.

The content of N in the absorber layer 14 is preferably from 30 to 60at. %. If the content of N is higher than this range, the film densitytends to be low, and the absorption coefficient of EUV light tends to below, whereby a sufficient EUV light absorption characteristic may not beobtained. Further, the acid resistance of the absorber layer 14 maydeteriorate.

The total content of Ta and Hf is more preferably from 45 to 80 at. %,further preferably from 45 to 75 at %. Further, the compositional ratioof Ta to Hf is more preferably from 7:3 to 4:6, further preferably from6.5:3.5 to 4.5:5.5, particularly preferably from 6:4 to 5:5. The Ncontent is more preferably from 20 to 55 at. %, further preferably from25 to 55 at. %.

The absorber layer 14 may contain from 0.1 to 1.0 at. % of Zr from thetarget used during the film deposition. It is preferred to suppress theamount of Zr to be low with a view to improving the EUV absorptionperformance.

Further, the absorber layer 14 preferably contains at most 20 at. % ofoxygen (O), whereby the etching rate will be good. The oxygen content isfurther preferably at most 10 at. %, particularly preferably at most 5at. %. Further, the absorber layer 14 preferably contains at most 10 at.% of carbon (C). The carbon content is more preferably 0 to at most 5at. %, particularly preferably 0 to at most 3 at. %.

However, taking into consideration a preferred etching rate, adhesion,etc. as the absorber layer, the absorber layer 14 preferably contains 0to at most 5 at. % of Cr.

Further, from the viewpoint of the absorption coefficient of EUV light,the total amount of at least one element selected from the groupconsisting of Si, Mo, B, Y, Zr, Nb, La and Ti is preferably 0 to at most10 at. %, further preferably 0 to at most 5 at. %.

Further, the above limitation of the amount of the additional metals isapplicable also to the low reflective layer which will be describedhereinafter.

With the absorber layer 14 being of the above-described construction,its crystalline state is preferably amorphous. In this specification,“the crystalline state is amorphous” includes one having a finecrystalline structure in addition to one having an amorphous structurewith no crystalline structure at all. When the absorber layer 14 is afilm of an amorphous structure or a film of a fine crystallinestructure, the surface of the absorber layer 14 is excellent insmoothness.

In the EUV mask blank 1 of the present invention, it is preferred thatthe absorber layer 14 is a film of an amorphous structure or a film of afine crystalline structure, whereby the surface roughness (rms) of theabsorber layer 14 is at most 0.5 nm. Here, the surface roughness of theabsorber layer 14 may be measured by means of an atomic forcemicroscope. If the surface roughness of the absorber layer 14 is large,the edge roughness of a pattern formed in the absorber layer 14 tends tobe large, whereby the dimensional precision of the pattern tends todeteriorate. As the pattern becomes fine, the influence of the edgeroughness becomes distinct, and therefore, the surface of the absorberlayer 14 is required to be smooth.

When the surface roughness (rms) of the absorber layer 14 is at most 0.5nm, the surface for the absorber layer 14 is sufficiently smooth,whereby there will be no possibility of deterioration of the dimensionalprecision of a pattern due to an influence of the edge roughness. Thesurface roughness (rms) of the absorber layer 14 is more preferably atmost 0.4 nm, further preferably at most 0.3 nm.

Further, the crystalline state of the absorber layer 14 being amorphous,i.e. being of an amorphous structure or a fine crystalline structure,can be ascertained by an X-ray diffraction (XRD) method. When thecrystalline state of the absorber layer 14 is of an amorphous structureor a fine crystalline structure, no sharp peak is observed among thediffraction peaks obtainable by the XRD measurement.

The thickness of the absorber layer 14 is preferably from 50 to 200 nm,more preferably from 50 to 100 nm.

The absorber layer 14 of the above construction can be formed bycarrying out a sputtering method using a TaHf compound target, forexample, magnetron sputtering or ion beam sputtering.

Here, in a case where the absorber layer 14 contains no N i.e. containsonly Ta and Hf, the absorber layer 14 is formed by discharging a TaHfcompound target in an inert gas atmosphere, for example, in an argon(Ar) atmosphere.

On the other hand, in a case where the absorber layer 14 contains N i.e.contains Ta, Hf and N, the absorber layer 14 is formed by discharging aTaHf compound target in a nitrogen (N₂) atmosphere diluted with argon.

The TaHf compound target preferably has a composition comprising Ta=30to 70 at. % and Hf=70 to 30 at. %, whereby an absorber layer having adesired composition can be obtained, and it is thereby possible to avoidfluctuation in the film composition or film thickness. The TaHf compoundtarget may contain from 0.1 to 5.0 at. % of Zr.

Here, a TaHf compound target containing Hf having a low electricalresistivity is used, whereby, as is different from a case where a TaBcompound target containing B having a high electrical resistivity andhigh insulating property, is used, the film deposition is very stable,and it is possible to control the film composition or film thicknesseasily.

To form the absorber layer 14 by the above-mentioned method,specifically, the method may be carried out under the following filmdeposition conditions.

Case Wherein Absorber Layer (Containing No N) is Formed

Sputtering gas: Ar gas (gas pressure: 1.0×10⁻¹ Pa to 50×10⁻¹ Pa,preferably 1.0×10⁻¹ Pa to 40×10⁻¹ Pa, more preferably 1.0×10⁻¹ Pa to30×10⁻¹ Pa)

Applied electrical power: 30 to 1,000 W, preferably 50 to 750 W, morepreferably 80 to 500 W

Film deposition rate: 2.0 to 60 nm/min, preferably 3.5 to 45 nm/min,more preferably 5 to 30 nm/min

Case Wherein Absorber Layer (containing N) is Formed

Sputtering gas: Mixed gas of Ar and N₂ (N₂ gas concentration: 5 to 80vol %, preferably 10 to 75 vol %, more preferably 20 to 70 vol %. Gaspressure: 1.0×10⁻¹ Pa to 50×10⁻¹ Pa, preferably 1.0×10⁻¹ Pa to 40×10⁻¹Pa, more preferably 1.0×10⁻¹ Pa to 30×10⁻¹ Pa)

Applied electric power: 30 to 1,000 W, preferably 50 to 750 W, morepreferably 80 to 500 W

Film deposition rate: 2.0 to 60 nm/min, preferably 3.5 to 45 nm/min,more preferably 5 to 30 nm/min.

The low reflective layer 15 is constituted by a film which shows a lowreflectance against an inspection light to be used for inspection of amask pattern. In the preparation of a EUV mask, after forming a patternin the absorber layer, inspection is carried out to ascertain whetherthe pattern is formed as designed. In such an inspection of a maskpattern, an inspection machine is usually used wherein light of about257 nm is used as an inspection light. Namely, inspection is carried outby the difference in the reflectance against such light of about 257 nm,specifically by the difference in the reflectance between the surfaceexposed by removal of the absorber layer 14 by formation of a patternand the surface of the absorber layer 14 remained without being removedby the formation of the pattern. Here, the former is the surface of thereflective layer 12 or the surface of the protective layer 13, usuallythe surface of the protective layer 13. Accordingly, if the differencein the reflectance between the surface of the protective layer 13 andthe surface of the absorber layer 14 against the wavelength of theinspection light is small, the contrast at the time of the inspectionbecomes poor, and no accurate inspection can be done.

The absorber layer 14 having the above-described construction has anextremely low EUV light reflectance and thus has an excellentcharacteristic as an absorber layer for an EUV mask blank 1, but wheninspected with the wavelength of the inspection light, the lightreflectance may not necessarily be said to be sufficiently low. As aresult, the difference between the reflectance on the surface of theabsorber layer 14 and the reflectance on the surface of the protectivelayer 13 at the wavelength of the inspection light may be small, and thecontrast at the time of inspection may not be sufficiently obtained. Ifthe contrast at the time of the inspection is not sufficiently obtained,defects in the pattern cannot sufficiently be identified in the maskinspection, and no accurate inspection of defects can be carried out.

In the EUV mask blank 1 of the present invention, by forming a lowreflective layer 15 against the inspection light is formed on theabsorber layer 14, the contrast at the time of the inspection will begood. In other words, the light reflectance at the wavelength of theinspection light will be very low. Specifically, when the surface of thelow reflective layer 15 is irradiated with light in a wavelength regionof the inspection light, the maximum light reflectance at the wavelengthof the inspection light is preferably at most 15%, more preferably atmost 10%, further preferably at most 5%.

When the light reflectance at the wavelength of the inspection light onthe surface of the low reflective layer 15 is at most 15%, the contrastat the time of the inspection is good. Specifically, the contrastbetween reflected light with the wavelength of the inspection light onthe surface of the protective layer 13 and reflected light with thewavelength of the inspection light on the surface of the low reflectivelayer 15, is at least 30%.

In this specification, the contrast can be obtained by using thefollowing formula (I).

Contrast(%)=((R ₂ −R ₁)/(R ₂ +R ¹)×100  formula (I)

Here, at the wavelength of the inspection light, R₂ is the reflectanceon the surface of the protective layer 13, and R₁ is the reflectance onthe surface of the low reflective layer 15. The above R₁ and R₂ aremeasured in a state where, as shown in FIG. 2, a pattern is formed inthe absorber layer 14 (and the low reflective layer 15) of an EUV maskblank 1 shown in FIG. 1. The above R₂ is a value measured at the surfaceof the reflective layer 12 or the surface of the protective layer 13exposed by removal of the absorber layer 14 and the low reflective layer15 by the formation of the pattern, and R₁ is a value measured at thesurface of the low reflective layer 15 remained without being removed bythe formation of the pattern, in FIG. 2.

In the present invention, the contrast represented by the above formula(1) is more preferably at least 45%, further preferably at least 60%,particularly preferably at least 80%.

In order to attain the above-described characteristic, the lowreflective layer 15 is preferably made of a material having a refractiveindex at the wavelength of the inspection light being lower than theabsorber layer 14, and its crystalline state is preferably amorphous.

With the low reflective layer 15 of the EUV mask blank 1 of the presentinvention, the above-described characteristic is attained by containingTa, Hf and oxygen (O) in a specific ratio which will be described below.

In the low reflective layer 15, it is preferred that the total contentof Ta and Hf is from 30 to 80 at. %, and the compositional ratio of Tato Hf is from 8:2 to 4:6. If the total content of Ta and Hf is less than30 at. %, the electrical conductivity of the low reflective layer 15tends to be low, and a problem of charge-up is likely to result at thetime of electron beam writing in the low reflective layer 15. If thetotal content of Ta and Hf exceeds 80 at. %, it tends to be difficult tosufficiently lower the light reflectance against light for inspection ofa pattern. Further, if Hf is lower than the above compositional ratio,the crystalline state of the low reflective layer 15 tends to be hardlyamorphous. If Hf in the low reflective layer 15 is higher than the abovecompositional ratio, the etching characteristic of the low reflectivelayer tends to deteriorate, and the required etching selective ratio maynot be satisfied.

In the low reflective layer 15, the content of O is preferably from 20to 70 at. %. If the content of O is lower than 20 at. %, it may beimpossible to sufficiently lower the light reflectance in the wavelengthregion of the light for inspection of a pattern. If the content of O ishigher than 70 at. %, the acid resistance of the low reflective layer 15tends to be low, and the insulating property of the low reflective layer15 tends to increase, whereby a problem may be likely such thatcharge-up takes place at the time of electron beam writing in the lowreflective layer 15.

The total content of Ta and Hf is more preferably from 35 to 80 at. %,further preferably from 35 to 75 at. %. Further, the compositional ratioof Ta to Hf is more preferably Ta:Hf=7:3 to 4:6, further preferably6.5:3.5 to 4.5:5.5, particularly preferably 6:4 to 5:5. the content of Ois more preferably from 20 to 65 at. %, further preferably from 25 to 65at. %.

Further, the low reflective layer 15 may contain an element other thanTa, Hf and O, as the case requires. In such a case, the element to beincorporated to the low reflective layer 15 is required to satisfysuitability as a mask blank, such as an absorption characteristic forEUV light.

Nitrogen (N) may be mentioned as an example of the element which may beincorporated to the low reflective layer 15. By the incorporation of Nto the low reflective layer 15, smoothness of the surface of the lowreflective layer 15 is expected to improve.

In a case where the low reflective layer 15 contains N, it is preferredthat a total content of Ta and Hf in the low reflective layer 15 is from30 to 80 at. %, and the compositional ratio of Ta to Hf is Ta:Hf=8:2 to4:6, the total content of N and O is from 20 to 70 at. %, and thecompositional ratio of N to O is from 9:1 to 1:9. Further, in thepresent invention, the compositional ratio of N to O means acompositional ratio of N to O by atomic ratio. If the total content ofTa and Hf is less than 30 at. %, the electrical conductivity of the lowreflective layer 15 tends to decrease and a charge-up problem may belikely at the time of electron beam writing in the low reflective layer15. If the total content of Ta and Hf exceeds 80 at. %, it will beimpossible to sufficiently lower the light reflectance against light forinspection of a pattern. If Hf in the low reflective layer 15 is lowerthan the above compositional ratio, the crystalline state of the lowreflective layer 15 may not be amorphous. If Hf in the low reflectivelayer 15 is higher than the above compositional ratio, the etchingcharacteristic of the low reflective layer tends to deteriorate, and therequired etching selective ratio may not be satisfied. Further, in acase where the content of N and O is lower than 20 at. %, the lightreflectance in a wavelength region of light for inspection of a patternmay not be made sufficiently low. In a case where the content of N and Ois higher than 70 at. %, the acid resistance of the low reflective layer15 tends to be low, and the insulating property of the low reflectivelayer 15 tends to increase, whereby a problem may be likely such thatcharge-up takes place at the time of electron beam writing in the lowreflective layer 15.

The total content of Ta and Hf is more preferably from 35 to 80 at. %,further preferably from 35 to 75 at. %. Further, the compositional ratioof Ta to Hf is more preferably Ta:Hf=7:3 to 4:6, further preferably6.5:3.5 to 4.5:5.5, particularly preferably 6:4 to 5:5. The totalcontent of N and O is more preferably from 20 to 65 at. %, furtherpreferably from 25 to 65 at. %.

When the low reflective layer 15 is of the above-described construction,its crystalline state is amorphous, and its surface is excellent insmoothness. Specifically, the surface roughness (rms) of the lowreflective layer 15 is preferably at most 0.5 nm.

As mentioned above, in order to prevent deterioration of the dimensionalprecision of a pattern due to an influence of edge roughness, thesurface of the absorber layer 14 is required to be smooth. The lowreflective layer 15 is formed on the absorber layer 14, and for the samereason, its surface is required to be smooth.

When the surface roughness (rms) of the surface of the low reflectivelayer 15 is at most 0.5 nm, the surface of the low reflective layer 15is sufficiently smooth, whereby there will be no possibility that thedimensional precision of a pattern will deteriorate due to an influenceof edge roughness. The surface roughness (rms) of the low reflectivelayer 15 is more preferably at most 0.4 nm, further preferably at most0.3 nm.

Further, with a view to reducing the surface roughness, it is preferredto incorporate N to the low reflective layer 15.

Further, the crystalline state of the low reflective layer 15 beingamorphous, i.e. being an amorphous structure or a fine crystallinestructure, may be ascertained by an X-ray diffraction (XRD) method. Whenthe crystalline state of the low reflective layer 15 is of an amorphousstructure or a fine crystalline structure, no sharp peak will beobserved among the diffraction peaks obtainable by the XRD measurement.

In a case where the low reflective layer 15 is formed on the absorberlayer 14, the total thickness of the absorber layer 14 and the lowreflective layer 15 is preferably from 55 to 130 nm. Further, if thethickness of the low reflective layer 15 is larger than the thickness ofthe absorber layer 14, the EUV light absorbing characteristic at theabsorber layer 14 is likely to deteriorate. Accordingly, the thicknessof the low reflective layer 15 is preferably less than the thickness ofthe absorber layer. Therefore, the thickness of the low reflective layer15 is preferably from 5 to 30 nm, more preferably from 10 to 20 nm.

The low reflective layer 15 of the above-described construction can beformed by carrying out a sputtering method using a TaHf compound target,for example, magnetron sputtering or ion beam sputtering.

Further, in a case where the low reflective layer 15 contains no N, i.e.contains Ta, Hf and O, the low reflective layer 15 is formed bydischarging a TaHf compound target in an oxygen (O₂) atmosphere dilutedby an inert gas such as argon. Otherwise, a TaHf compound target may bedischarged in an inert gas atmosphere to form a film containing Ta andHf, and then the formed film may be oxidized by e.g. exposing it tooxygen plasma or irradiating it with ion beams using oxygen, to obtain alow reflective layer 15 containing Ta, Hf and O.

On the other hand, in a case where the low reflective layer 15 containsN, i.e. contains Ta, Hf, O and N, the low reflective layer 15 is formedby discharging a TaHf compound target in an oxygen (O₂)/nitrogen (N₂)mixed gas atmosphere diluted with argon. Otherwise, a TaHf compoundtarget may be discharged in a nitrogen (N₂) atmosphere diluted withargon to form a film containing Ta, Hf and N, and then the formed filmis oxidized by exposing it to oxygen plasma or irradiating it with ionbeams using oxygen, to obtain a low reflective layer 15 containing Ta,Hf, O and N.

The TaHf compound target preferably has a composition comprising Ta=30to 70 at. % and Hf=70 to 30 at. %, whereby a low reflective layer havinga desired composition can be obtained, and it is possible to avoidfluctuation of the film composition or film thickness. The TaHf compoundtarget may contain from 0.1 to 5.0 at. % of Zr.

In order to form the absorber layer 14 by the above-described method,specifically, the method can be carried out under the following filmdeposition conditions.

Case Wherein Absorber Layer (Containing No N) is Formed

Sputtering gas: Ar gas (gas pressure: 1.0×10⁻¹ Pa to 50×10⁻¹ Pa,preferably 1.0×10⁻¹ Pa to 40×10⁻¹ Pa, more preferably 1.0×10⁻¹ Pa to30×10⁻¹ Pa)

Applied electric power: 30 to 1,000 W, preferably 50 to 750 W, morepreferably 80 to 500 W

Film deposition rate: 2.0 to 60 nm/min, preferably 3.0 to 45 nm/min,more preferably 5 to 30 nm/min

Case Wherein Low Reflective Layer (Containing No N) is Formed

Sputtering gas: Mixed gas of Ar and O₂ (O₂ gas concentration: 3 to 80vol %, preferably 5 to 60 vol %, more preferably 9 to 40 vol %. Gaspressure: 1.0×10⁻¹ Pa to 50×10⁻¹ Pa, preferably 1.0×10⁻¹ Pa to 40×10⁻¹Pa, more preferably 1.0×10⁻¹ Pa to 30×10⁻¹ Pa)

Applied electric power: 30 to 1,000 W, preferably 50 to 750 W, morepreferably 80 to 500 W

Film deposition rate: 2.0 to 60 nm/min, preferably 3.5 to 45 nm/min,more preferably 5 to 30 nm/min

Case Wherein Low Reflective Layer (Containing N) is Formed

Sputtering gas: Mixed gas of Ar, O₂ and N₂ (O₂ gas concentration: 5 to40 vol %, N₂ gas concentration: 5 to 40 vol %, preferably O₂ gasconcentration: 6 to 35 vol %, N₂ gas concentration: 6 to 35 vol %, morepreferably O₂ gas concentration: 10 to 30 vol %, N₂ gas concentration:10 to 30 vol %. Gas pressure: 1.0×10⁻¹ Pa to 50×10⁻¹ Pa, preferably1.0×10⁻¹ Pa to 40×10⁻¹ Pa, more preferably 1.0×10⁻¹ Pa to 30×10⁻¹ Pa)

Applied electric power: 30 to 1,000 W, preferably 50 to 750 W, morepreferably 80 to 500 W

Film deposition rate: 2.0 to 60 nm/min, preferably 3.5 to 45 nm/min,more preferably 5 to 30 nm/min

Further, in the EUV mask blank 1 of the present invention, it ispreferred to form the low reflective layer 15 on the absorber layer 14,because the wavelength of light for inspection of a pattern is differentfrom the wavelength of EUV light. Accordingly, in a case where EUV light(in the vicinity of 13.5 nm) is used as light for inspection of apattern, it is considered unnecessary to form a low reflective layer 15on the absorber layer 14. The wavelength of the inspection light tendsto shift to the short wavelength side as the pattern dimension becomessmall and in future, it may shift to 193 nm or further shift to 13.5 nm.

The EUV mask blank 1 of the present invention may have a functional filmknown in the field of EUV mask blanks, in addition to the reflectivelayer 12, the protective layer 13, the absorber layer 14 and the lowreflective layer 15. As a specific example of such a functional film, ahigh dielectric coating may be mentioned which is applied on the rearside of a substrate in order to accelerate electrostatic chucking of thesubstrate, as disclosed in e.g. JP-A-2003-501823. Here, in the substrate11 in FIG. 1, the rear side of the substrate means the surface on theside opposite to the side on which the reflective layer 12 is formed.For the high dielectric coating to be provided on the rear side of thesubstrate for such a purpose, the electrical conductivity and thethickness of the constituting material are selected so that the sheetresistance will be at most 100Ω/□. The constituting material for thehigh dielectric coating may be selected widely from those disclosed inknown literatures. For example, a high dielectric coating disclosed inJP-A-2003-501823, specifically, a coating comprising silicon, TiN,molybdenum, chromium and TaSi, may be applied. The thickness of the highdielectric coating may, for example, be from 10 to 1,000 nm.

The high dielectric coating may be formed by using a known filmdeposition method, for example, a sputtering method such as magnetronsputtering or ion beam sputtering, a CVD method, a vacuum depositionmethod or an electrolytic plating method.

EXAMPLES

Now, the present invention will be described in further detail withreference to Examples and Comparative Examples.

Example 1

In this Example, an EUV mask blank 1 shown in FIG. 1 was prepared.However, in the EUV mask blank 1 in Example 1, a low reflective layer 15was not formed on an absorber layer 14. As a substrate 11 for filmdeposition, a SiO₂—TiO₂ glass substrate (size: 6 inch (152 mm) square,thickness: 6.3 mm) was used. This glass substrate has a thermalexpansion coefficient of 0.2×10⁻⁷/° C., a Young's modulus of 67 GPa, aPoisson ratio of 0.17 and a specific rigidity of 3.07×10⁷ m²/s². Thisglass substrate was formed by polishing to have a smooth surface with asurface roughness (rms) of at most 0.15 nm and to a flatness of at most100 nm.

On the rear side of the substrate 11, a Cr film having a thickness of100 nm was deposited by means of magnetron sputtering to provide a highdielectric coating having a sheet resistance of 100 Ω/□.

To a flat plate-shaped usual electrostatic chuck, the substrate 11(size: 6 inch (152 mm) square, thickness: 6.3 mm) was fixed by means ofthe formed Cr film, and on the surface of the substrate 11, a Si filmand a Mo film were alternately deposited by ion beam sputtering for 40cycles, to form a reflective Si/Mo multilayer film (reflective layer 12)having a total film thickness of 272 nm ((4.5 nm+2.3 nm)×40).

Further, on the reflective Si/Mo multilayer film (reflective layer 12),a Ru film (film thickness: 2.5 nm) was deposited by ion beam sputteringto form a protective layer 13.

The film deposition conditions for the Si film, the Mo film and the Rufilm were as follows.

Film Deposition Conditions for Si Film

Target: Si target (boron-doped)

Sputtering gas: Ar gas (gas pressure: 0.02 Pa)

Voltage: 700 V

Film deposition rate: 0.077 nm/sec

Film thickness: 4.5 nm

Film Deposition Conditions for Mo Film

Target: Mo target

Sputtering gas: Ar gas (gas pressure: 0.02 Pa)

Voltage: 700 V

Film deposition rate: 0.064 nm/sec

Film thickness: 2.3 nm

Film Deposition Conditions for Ru Film

Target: Ru target

Sputtering gas: Ar gas (gas pressure: 0.02 Pa)

Voltage: 700 V

Film deposition rate: 0.023 nm/sec

Film thickness: 2.5 nm

Then, on the protective layer 13, an absorber layer 14 (TaHf film)containing Ta and Hf was formed by magnetron sputtering to obtain an EUVmask blank 1 having the reflective layer 12, the protective layer 13 andthe absorber layer 14 formed in this order on the substrate 11.

The film deposition conditions for the absorber layer 14 were asfollows.

Film Deposition Conditions for Absorber Layer 14 (TaHf Film)

Target:TaHf compound target (compositional ratio: Ta 55 at. %, Hf 45 at.%)

Sputtering gas: Ar gas (gas pressure: 0.3 Pa)

Applied electric power: 150 W

Film deposition rate: 0.29 nm/sec

Film thickness: 60 nm

The following evaluations (1) to (4) were carried out with respect tothe absorber layer of the EUV mask blank obtained by the aboveprocedure.

(1) Film Composition

The composition of the absorber layer 14 (TaHf film) was measured byusing an X-ray photoelectron spectrometer (Number 5500, manufactured byPERKIN ELEMER-PHI). The compositional ratio (at. %) of the absorberlayer 14 (TaHf film) was Ta:Hf=55:45 (content of Ta: 55 at. %, contentof Hf: 45 at. %). The content of Zr was from 0.3 to 0.7 at. %.

(2) Crystalline State

The crystalline state of the absorber layer 14 (TaHf film) wasascertained by an X-ray diffractometer (manufactured by RigakuCorporation). No sharp peak was observed among the obtained diffractionpeaks, whereby it was confirmed that the crystalline state of theabsorber layer 14 (TaHf film) was of an amorphous structure or a finecrystalline structure.

(3) Surface Roughness

The surface roughness of the absorber layer 14 (TaHf film) was measuredby a dynamic force mode by using an atomic force microscope (SPI-3800,manufactured by SII). The measuring region for the surface roughness was1 μm×1 μm, and as the cantilever, SI-DF40 (manufactured by SII) wasused.

The surface roughness (rms) of the absorber layer was 0.10 nm.

(4) Resistance Value

The resistance value of the absorber layer 14 (TaHf film) was measuredby using a four-point probe meter (Loresta APMCP-T400, manufactured byMitsubishi Yuka K.K.) and was found to be 1.8×10⁻⁴ Ω·cm.

Example 2

In this Example, an EUV mask blank 1 was prepared wherein a lowreflective layer 15 (TaHfON film) containing Ta, Hf, O and N is formedon an absorber layer 14.

In this Example, the procedure to form an absorber layer 14 on aprotective layer 13 was carried out in the same manner as in Example 1.On the absorber layer 14, a low reflective layer (TaHfON film)containing Ta, Hf, O and N was formed by magnetron sputtering, as a lowreflective layer 15 against an inspection light having a wavelength of257 nm. The compositional ratio (at. %) of the low reflective layer wasmeasured by the same method as in Example 1 and found to beTa:Hf:N:O=35:15:15:35.

The film deposition conditions for the low reflective layer 15 (TaHfONfilm) were as follows.

Film Deposition Conditions for Low Reflective Layer 15 (TaHfON film)

Target:TaHf compound target (compositional ratio: Ta 55 at. %, Hf 45 at.%)

Sputtering gas: Mixed gas of Ar, N₂ and O₂ (Ar: 45 vol %, N₂: 23 vol %,O₂: 32 vol %, gas pressure: 0.3 Pa)

Applied electric power: 150 W

Film deposition rate: 0.13 nm/sec

Film thickness: 10 nm

The following evaluation (5) was carried out with respect to the lowreflective layer 15 (TaHfON film) of the EUV mask blank obtained by theabove procedure.

(5) Reflection Characteristic (Evaluation of Contrast)

In Example 1, at a stage where up to the protective layer 13 (Ru film)was formed, the reflectance on the surface of the protective layer 13against the light (wavelength: 257 nm) for inspection of a pattern wasmeasured by using a spectrophotometer. Further, in Example 2, afterforming the low reflective layer 15 (TaHfON film), the reflectance onthe surface of the low reflective layer against the light for inspectionof a pattern was measured. As a result, the reflectance on the surfaceof the protective layer 13 was 60.0%, and the reflectance on the surfaceof the low reflective layer 15 (TaHfON film) was 1.8%. Using suchresults and the above-mentioned formula, the contrast was obtained andfound to be 94.1%.

With respect to the obtained EUV mask blank, the reflectance against EUVlight was measured by irradiating the surface of the low reflectivelayer 15 (TaHfON film) with EUV light (wavelength: 13.5 nm). As aresult, the reflectance against EUV light was 0.4%, whereby it wasconfirmed that the mask blank was excellent in the EUV absorptioncharacteristic.

Further, the etching characteristic of the absorber layer (TaHf film) ofthe EUV mask blank obtained by the above procedure was evaluated asfollows.

(6) Etching Characteristic

With respect to the etching characteristic, evaluation was carried outby the following method instead of evaluation by means of the EUV maskblank prepared by the above procedure.

On a test sample table (4 inch quartz base plate) of a RF plasma etchingapparatus, a Si chip (10 mm×30 mm) having a Ru film or a TaHf filmdeposited thereon by the following method, was set as a test sample. Inthis state, the Ru film or the TaHf film of the Si chip set on the testsample table was subjected to plasma RF etching under the followingconditions.

Bias RF: 50 W

Etching time: 120 sec

Trigger pressure: 3 Pa

Etching pressure: 1 Pa

Etching gas: Cl₂/Ar

Gas flow rate (Cl₂/Ar): 20/80 sccm

Distance between electrode substrates: 55 mm

The film deposition of the Ru film was carried out under the followingfilm deposition conditions by magnetron sputtering.

Film Deposition Conditions for Ru Film

Target: Ru target

Sputtering gas: Ar gas (gas pressure: 0.3 Pa)

Output: 150 W

Film deposition rate: 0.25 nm/sec

Film thickness: 2.5 nm

The TaHf film was deposited by discharging a TaHf compound target in anAr atmosphere by means of magnetron sputtering. Here, the filmdeposition was carried out under the following two types of conditions.

Film Deposition Conditions (1) FOR TaHf Film

Target:TaHf compound target (compositional ratio: Ta 55 at. %, Hf 45 at.%)

Sputtering gas: Ar gas (gas pressure: 0.3 Pa)

Applied electric power: 150 W

Film deposition rate: 0.29 nm/sec

Film thickness: 60 nm

Film Deposition Conditions (2) for TaHf Film

Target:TaHf compound target (compositional ratio: Ta 45 at. %, Hf 55 at.%)

Sputtering gas: Ar gas (gas pressure: 0.3 Pa)

Applied electric power: 150 W

Film deposition rate: 0.35 nm/sec

Film thickness: 60 nm

With respect to the Ru film and the TaHf films (1) and (2) depositedunder the above conditions, the etching rates were obtained, and theetching selective ratios were obtained by the following formula.

Etching selective ratio=Etching rate of TaHf film/etching rate of Rufilm

The etching selective ratios of the TaHf films (1) and (2) were asfollows.

TaHf Film (1)

Etching rate of TaHf film: 19.8 (nm/min)

Etching rate of Ru film: 1.48 (nm/min)

Etching selective ratio: 13.3

TaHf Film (2)

Etching rate of TaHf film: 19.0 (nm/min)

Etching rate of Ru film: 1.48 (nm/min)

Etching selective ratio: 12.8

The etching selective ratio to the protective layer 13 is preferably atleast 10, and each of the TaHf films (1) and (2) had a sufficientetching selective ratio. Further, the TaHf films (1) and (2) haveetching rates which are high as compared with a TaBN film in ComparativeExample 1 given hereinafter and thus are expected to have an effect toreduce a damage to the resist during etching. Further, due to thereduction of the damage to the resist, reduction of the film thicknessof the resist is expected.

Example 3

In this Example, an EUV mask blank 1 was prepared wherein a lowreflective layer 15 (TaHfO film) containing Ta, Hf and O was formed onan absorber layer 14.

In this Example, the procedure up to forming an absorber layer 14 on aprotective layer 13 was carried out in the same manner as in Example 1.On an absorber layer 14, a low reflective layer (TaHfO film) containingTa, Hf and O was formed by magnetron sputtering as a low reflectivelayer 15 against an inspection light having a wavelength of 257 nm. Thecompositional ratio (at. %) of the low reflective layer was measured inthe same manner as in Example 1 and found to be Ta:Hf:O=40:20:40.

The film deposition conditions for the low reflective layer 15 (TaHfOfilm) were as follows.

Film Deposition Conditions for Low Reflective Layer 15 (TaHfO Film)

Target:TaHf compound target (compositional ratio: Ta 55 at. %, Hf 45 at.%)

Sputtering gas: Mixed gas of Ar and O₂ (Ar: 70 vol %, O₂: 30 vol %, gaspressure: 0.3 Pa)

Applied electric power: 150 W

Film deposition rate: 0.43 nm/sec

Film thickness: 10 nm

Evaluation of the contrast of the obtained low reflective layer 15(TaHfO film) was carried out in the same manner as in Example 2. As aresult, the reflectance on the surface of the protective layer 13 was60.0%, and the reflectance on the surface of the low reflective layer 15(TaHfO film) was 2.6%. The contrast was obtained by using such resultsand the above formula, and found to be 91.7%.

Further, the EUV light reflectance on the surface of the low reflectivelayer 15 (TaHfO film) was measured. As a result, the EUV lightreflectance was 0.4%, whereby it was confirmed that the layer wasexcellent in the EUV absorption characteristic.

Comparative Example 1

Comparative Example 1 was carried out in the same manner as in Example 1except that the absorber layer was a film of a nitride of tantalum/boronalloy (TaBN). The TaBN film was deposited under the following conditionsby using a TaB target (Ta:B=50 at. %:50 at. %).

Film Deposition Conditions for TaBN Layer

Target:TaB target (compositional ratio: Ta 50 at. %, B 50 at. %)

Sputtering gas: Ar gas, N₂ gas (Ar: 86 vol %, N₂: 14 vol %, gaspressure: 0.3 Pa)

Applied electric power: 150 W

Film deposition rate: 0.05 nm/sec

Film thickness: 60 nm

The composition (at. %) of the obtained TaBN film was measured by usingan X-ray photoelectron spectrometer, whereby the content of B was atleast 5 at. %.

The crystalline state of the TaBN film after the film deposition wasconfirmed by an X-ray diffraction apparatus, whereby no sharp peak wasobserved among the obtained diffraction peaks, and thus, the crystallinestate of the absorber layer was confirmed to be of an amorphousstructure or a fine crystalline structure.

Further, the surface roughness (rms) of the TaBN film after the filmdeposition was confirmed by the same method as in Example 1 and wasfound to be 0.2 nm.

Further, on the absorber layer 14, an oxynitride of a tantalum/boronalloy (TaBON) was formed as a low reflective layer, and the patterninspection light (wavelength: 257 nm) reflectance on the surface of theprotective layer 13 (Ru film) and the TaBON layer was measured in thesame manner as in Example 2. The TaBON film was deposited under thefollowing conditions by using a TaB target (Ta:B=50 at. %:50 at. %).

Film Deposition Conditions for TaBON Layer

Target:TaB target (compositional ratio: Ta 50 at. %, B 50 at. %)

Sputtering gas: Ar gas, N₂ gas, O₂ gas (Ar: 60 vol %, N₂: 20 vol %, O₂:20 vol %, gas pressure: 0.3 Pa)

Applied electric power: 150 W

Film deposition rate: 0.05 nm/sec

Film thickness: 10 nm

As a result, the reflectance on the surface of the absorber layer 14 was60.0%, and the reflectance on the surface of the TaBON layer was 9.9%.The contrast was obtained by using such results and the above formulaand was found to be 71.7%, and thus, the contrast was confirmed to below as compared with Example 2.

With respect to the TaBN film, the etching characteristic was evaluatedin the same manner as mentioned above. As a result, the etchingselective ratio of the TaBN film was 10.4 (etching rate of TaBN film:15.4 (nm/min), etching rate of Ru film: 1.48 (nm/min)).

Further, the film deposition rate for the TaBN layer in ComparativeExample 1 was substantially slow at a level of about ⅙ of the filmdeposition rate in Example 1. Further, in order to confirm thereproducibility, the operation was repeated a plurality of times underthe conditions of Comparative Example 1, whereby it was confirmed thatthe discharge was instable, there was a case where film deposition wasimpossible, and it was very difficult to control the film composition orthe film thickness.

Comparative Example 2

Comparative Example 2 is carried out in the same manner as in Example 1except that the Hf content in the absorber layer (TaHf film) is lessthan 20 at. % (e.g. 10 at. %). Here, the absorber layer (TaHf film)having a Hf content of less than 20 at. %, is formed by carrying outmagnetron sputtering by using a TaHf compound target having a Hf contentof less than 20 at. %.

The crystalline state of the obtainable absorber layer (TaHf film) isconfirmed by using an X-ray diffraction apparatus, whereby sharp peaksare observed among the obtained diffraction peaks, whereby it isconfirmed that the absorber layer (TaHf film) is crystalline. Further,the surface roughness (rms) is 0.6 nm.

Comparative Example 3

Comparative Example 3 is carried out in the same manner as in Example 1except that the Hf content in the absorber layer (TaHf film) exceeds 60at. % (e.g. 70 at. %). Here, the absorber layer (TaHf film) having a Hfcontent exceeding 60 at. %, is formed by carrying out magnetronsputtering by using a TaHf compound target having a Hf content exceeding60 at. %.

The crystalline state of the obtainable absorber layer (TaHf film) isconfirmed by using an X-ray diffraction apparatus, whereby no sharp peakis observed among the obtained diffraction peaks, whereby it is possibleto confirm that the absorber layer (TaHf film) is of an amorphousstructure or a fine crystalline structure. Further, the surfaceroughness (rms) is also 0.1 nm.

With respect to the absorber layer (TaHf film), the etchingcharacteristic is evaluated in the same manner as in Example 2. As aresult, it is confirmed that the etching selective ratio for the TaHffilm is at most 5 (etching rate of the TaHf film: 5.18 (nm/min), theetching rate of the Ru film: 1.48 (nm/min)), and thus, the etchingselective ratio is low as compared with Example 2.

INDUSTRIAL APPLICABILITY

The EUV mask blank of the present invention has an absorber layer whichhas a low reflectance particularly in a wavelength region of EUV lightand pattern inspection light and which is easy to control to have adesired layer composition and thickness, and thus, it is useful for theproduction of photomasks in the semiconductor industry. The photomaskmay be used to pattern a photoresist layer followed by etching exposedsurfaces of an underlying surface such as a semiconductor material, adielectric material or a metalization layer.

The entire disclosure of Japanese Patent Application No. 2006-350932filed on Dec. 27, 2006 including specification, claims, drawings andsummary is incorporated herein by reference in its entirety.

1. A reflective mask blank for EUV lithography, which comprises asubstrate, and a reflective layer to reflect EUV light and an absorberlayer to absorb EUV light, formed in this order on the substrate,wherein the absorber layer comprises tantalum (Ta) and hafnium (Hf), andin the absorber layer, the content of Hf is from 20 to 60 at. % and thecontent of Ta is from 40 to 80 at. %.
 2. A reflective mask blank for EUVlithography, which comprises a substrate, and a reflective layer toreflect EUV light and an absorber layer to absorb EUV light, formed inthis order on the substrate, wherein the absorber layer comprisestantalum (Ta), hafnium (Hf) and nitrogen (N), and in the absorber layer,the total content of Ta and Hf is from 40 to 70 at. %, the compositionalratio of Ta to Hf is Ta:Hf=8:2 to 4:6, and the content of N is from 30to 60 at. %.
 3. The reflective mask blank for EUV lithography accordingto claim 1, wherein in the absorber layer, a total content of B, Si andGe is 0 to at most 5 at. %.
 4. The reflective mask blank for EUVlithography according to claim 2, wherein in the absorber layer, a totalcontent of B, Si and Ge is 0 to at most 5 at. %.
 5. The reflective maskblank for EUV lithography according to claim 1, wherein the absorberlayer contains from 0.1 to 1.0 at. % of Zr.
 6. The reflective mask blankfor EUV lithography according to claim 2, wherein the absorber layercontains from 0.1 to 1.0 at. % of Zr.
 7. The reflective mask blank forEUV lithography according to claim 1, wherein the absorber layercontains 0 to at most 5 at. % of Cr.
 8. The reflective mask blank forEUV lithography according to claim 2, wherein the absorber layercontains 0 to at most 5 at. % of Cr.
 9. The reflective mask blank forEUV lithography according to claim 1, wherein the crystalline state ofthe absorber layer is amorphous.
 10. The reflective mask blank for EUVlithography according to claim 2, wherein the crystalline state of theabsorber layer is amorphous.
 11. The reflective mask blank for EUVlithography according to claim 1, wherein the absorber layer has asurface roughness (rms) of at most 0.5 nm at its surface.
 12. Thereflective mask blank for EUV lithography according to claim 2, whereinthe absorber layer has a surface roughness (rms) of at most 0.5 nm atits surface.
 13. The reflective mask blank for EUV lithography accordingto claim 1, wherein the absorber layer has a thickness of from 50 to 200nm.
 14. The reflective mask blank for EUV lithography according to claim2, wherein the absorber layer has a thickness of from 50 to 200 nm. 15.The reflective mask blank for EUV lithography according to claim 1,further comprising a low reflective layer against an inspection light tobe used for inspection of a mask pattern formed on the absorber layer,the low reflective layer comprising tantalum (Ta), hafnium (Hf) andoxygen (O), and in the low reflective layer, the total content of Ta andHf is from 30 to 80 at. %, the compositional ratio of Ta to Hf is from8:2 to 4:6, and the content of 0 is from 20 to 70 at. %.
 16. Thereflective mask blank for EUV lithography according to claim 2, furthercomprising a low reflective layer against an inspection light to be usedfor inspection of a mask pattern formed on the absorber layer, the lowreflective layer comprising tantalum (Ta), hafnium (Hf) and oxygen (O),and in the low reflective layer, the total content of Ta and Hf is from30 to 80 at. %, the compositional ratio of Ta to Hf is from 8:2 to 4:6,and the content of 0 is from 20 to 70 at. %.
 17. The reflective maskblank for EUV lithography according to claim 1, further comprising a lowreflective layer against an inspection light to be used for inspectionof a mask pattern formed on the absorber layer, the low reflective layercomprising tantalum (Ta), hafnium (Hf), oxygen (O) and nitrogen (N), andin the low reflective layer, the total content of Ta and Hf is from 30to 80 at. %, the compositional ratio of Ta to Hf is Ta:Hf=8:2 to 4:6,the total content of N and O is from 20 to 70 at. %, and thecompositional ratio of N to O is N:O=9:1 to 1:9.
 18. The reflective maskblank for EUV lithography according to claim 2, further comprising a lowreflective layer against an inspection light to be used for inspectionof a mask pattern formed on the absorber layer, the low reflective layercomprising tantalum (Ta), hafnium (Hf), oxygen (O) and nitrogen (N), andin the low reflective layer, the total content of Ta and Hf is from 30to 80 at. %, the compositional ratio of Ta to Hf is Ta:Hf=8:2 to 4:6,the total content of N and O is from 20 to 70 at. %, and thecompositional ratio of N to O is N:O=9:1 to 1:9.
 19. A reflective maskblank for EUV lithography, which comprises a substrate, and a reflectivelayer to reflect EUV light, an absorber layer to absorb EUV light and alow reflective layer against an inspection light to be used forinspection of a mask pattern, formed in this order on the substrate,wherein the low reflective layer comprises tantalum (Ta), hafnium (Hf)and oxygen (O), and in the low reflective layer, the total content of Taand Hf is from 30 to 80 at. %, the compositional ratio of Ta to Hf isfrom 8:2 to 4:6, and the content of 0 is from 20 to 70 at. %.
 20. Areflective mask blank for EUV lithography, which comprises a substrate,and a reflective layer to reflect EUV light, an absorber layer to absorbEUV light and a low reflective layer to an inspection light to be usedfor inspection of a mask pattern, formed in this order on the substrate,wherein the low reflective layer comprises tantalum (Ta), hafnium (Hf),oxygen (O) and nitrogen (N), and in the low reflective layer, the totalcontent of Ta and Hf is from 30 to 80 at. %, the compositional ratio ofTa to Hf is Ta:Hf=8:2 to 4:6, the total content of N and O is from 20 to70 at. %, and the compositional ratio of N to 0 is N:O=9:1 to 1:9. 21.The reflective mask blank for EUV lithography according to claim 15,further comprising a protective layer to protect the reflective layerduring formation of a pattern in the absorber layer formed between thereflective layer and the absorber layer, and the contrast between lightreflected on the surface of the protective layer and light reflected onthe surface of the low reflective layer at a wavelength of light to beused for inspection of the pattern formed in the absorber layer, is atleast 30%.
 22. The reflective mask blank for EUV lithography accordingto claim 16, further comprising a protective layer to protect thereflective layer during formation of a pattern in the absorber layerformed between the reflective layer and the absorber layer, and thecontrast between light reflected on the surface of the protective layerand light reflected on the surface of the low reflective layer at awavelength of light to be used for inspection of the pattern formed inthe absorber layer, is at least 30%.
 23. The reflective mask blank forEUV lithography according to claim 21, wherein the protective layer isformed of any one selected from the group consisting of Ru, a Rucompound, SiO₂ and CrN.
 24. The reflective mask blank for EUVlithography according to claim 22, wherein the protective layer isformed of any one selected from the group consisting of Ru, a Rucompound, SiO₂ and CrN.
 25. The reflective mask blank for EUVlithography according to claim 15, wherein the reflectance on thesurface of the low reflective layer at a wavelength of light to be usedfor inspection of the pattern formed in the absorber layer, is at most15%.
 26. The reflective mask blank for EUV lithography according toclaim 16, wherein the reflectance on the surface of the low reflectivelayer at a wavelength of light to be used for inspection of the patternformed in the absorber layer, is at most 15%.
 27. The reflective maskblank for EUV lithography according to claim 1, wherein the absorberlayer is formed by carrying out a sputtering method using a target madeof a TaHf compound.
 28. The reflective mask blank for EUV lithographyaccording to claim 2, wherein the absorber layer is formed by carryingout a sputtering method using a target made of a TaHf compound in anatmosphere containing nitrogen.
 29. The reflective mask blank for EUVlithography according to claim 27, wherein the target made of a TaHfcompound contains from 0.1 to 5.0 at. % of Zr.
 30. The reflective maskblank for EUV lithography according to claim 28, wherein the target madeof a TaHf compound contains from 0.1 to 5.0 at. % of Zr.
 31. A photomaskcomprising a substrate, and a reflective layer to reflect EUV light anda patterned absorber layer to absorb EUV light, formed in this order onthe substrate, wherein the patterned absorber layer comprises tantalum(Ta) and hafnium (Hf), and in the patterned absorber layer, the contentof Hf is from 20 to 60 at. % and the content of Ta is from 40 to 80 at.%.
 32. A method of patterning a surface comprising: i) irradiating aphotoresist layer overlying a surface through the photomask of claim 31;ii) developing said photoresist layer to form a patterned photoresistand exposing surfaces of said surface; and iii) etching said exposedsurfaces of said surface.